1. Field of the Invention
This invention pertains, in general, to the field of retractable roofs for large structures, such as athletic stadiums. More specifically, the invention relates to an improved roof assembly that is optimal in terms of weight and bulk, that quickly adapts to maintain system alignment and balance during operation, that possesses fail-safe redundancy and that is economical to construct and to operate in comparison to conventional convertible stadium designs.
2. Description of the Related Technology
It is now common for athletic stadiums to be constructed with retractable roofs, because this type of construction offers spectators the pleasure of being outdoors on nice days, while providing shelter when necessary against extreme temperatures and inclement weather conditions. A retractable roof also can make possible the growth of natural grass within the stadium, which is often felt to be desirable in professional and major collegiate athletics.
A number of factors must be taken into account in the design of a stadium that has a retractable roof. For instance, the forces created by the exertion of natural forces such as wind, rain, snow and even earthquakes on such a large structure can be enormous, and the roof, the underlying stadium structure and the transport mechanism that is used to guide and move the roof between its retracted and operational positions must be engineered to withstand the worst possible confluence of such forces. Wind forces, for example, not only can impart tremendous displacement and lifting forces to a movable roof component, they can create potentially destructive vibration as well.
In addition, for reasons that are both aesthetic and practical, it is desirable to make the structural elements of the roof and the transport mechanism as unobtrusive and as space-efficient as possible. It is also desirable to make the roof structure and the transport mechanism as lightweight as possible, both to minimize the amount of energy that is necessary to open and close the roof structure and to minimize the need for additional structural reinforcement in the roof structure and in the underlying stadium structure.
Movable roof panels for large structures such as stadiums are still inevitably quite large and heavy and therefore present unique engineering challenges that are quite different than those that are faced by designers of smaller systems. For example, roof panels that are hundreds of feet in dimension undergo significant thermal expansion and contraction both on a macroscopic level as a result of atmospheric temperature conditions and on a more local level as a result of sunlight gradients, convection within and outside the stadium and so forth. For roof panels that are mounted for movement on trolleys or bearings that are significant distances from each other, thermal expansion and contraction present a significant engineering problem that is not faced by designers of smaller systems. Settling and shifting of the stadium and its foundation over time can also contribute to misalignment of large movable systems within the stadium such as roof panels. Maintaining the alignment of such systems during operation and while the systems are at rest is also an important consideration and presents challenges that are not present in smaller scale systems, especially when considered in conjunction with the external forces (wind shear, etc.) to which stadium roof panels are regularly subjected. It is desirable, of course, to minimize the mass and the weight of the bearing structure and the drive train that is used to support, reinforce and to move the movable roof panels between the opening and closed positions.
A need exists for an improved convertible stadium that is optimal in terms of weight and bulk, that quickly adapts to maintain system alignment and balance during operation, that possesses fail-safe redundancy and that is economical to construct and to operate in comparison to conventional convertible stadium designs.